feat(engine): swap ACCENT_PRIMARY from cyan #6fc2ef to brick red #a54242

Project-wide palette shift at user request. Replaces the cyan
primary accent everywhere it surfaces — splash boot screen,
home menu glyphs, action chevrons, replay overlay banner +
scrub fill + chip border, achievement checkmarks, leaderboard
#1 indicator, radial menu fill, focus ring, card-back canonical
badge, etc. — with `#a54242` from the same base16-eighties
family as the existing pink suit colour.

Knock-on changes that all land in this commit per the
lockstep rule:

- ui_theme.rs: ACCENT_PRIMARY (#a54242), ACCENT_PRIMARY_HOVER
  (#c25e5e brightened companion), FOCUS_RING (same hue, 0.85
  alpha). Module-level palette comment + STOCK_BADGE_FG +
  CARD_SHADOW_ALPHA_DRAG doc strings updated to match.
- card_plugin.rs: card_back_colour(0) now returns the brick-red
  ACCENT_PRIMARY (was cyan). RED_SUIT_COLOUR_CBM swapped from
  cyan to lime #acc267 — the CBM alternative needs to stay
  hue-distinct from the new red-family primary, lime is the
  next-best non-red base16-eighties accent. text_colour doc
  + CBM tests renamed cyan→lime in lockstep
  (text_colour_color_blind_mode_swaps_red_suits_to_lime).
- card_face_svg.rs: BACK_ACCENTS[0] now "#a54242" (canonical
  Terminal back).
- splash_plugin.rs / ui_modal.rs / replay_overlay.rs /
  selection_plugin.rs: descriptive "cyan" comments swapped to
  "accent" / "primary-accent" wording so the doc strings stay
  decoupled from any specific hue. Future palette tweaks won't
  require comment churn.
- design-system.md: YAML token frontmatter updated (primary,
  surface-tint, suit-red-cb, primary-container,
  on-primary-container, inverse-primary). Palette table gains
  a project-specific `base08` slot for the new red. CTA /
  Selection / Card-back badge / Primary button / Bottom-bar
  active-icon / glow / CBM swap text all retuned. Historical
  references preserved (e.g. "Was cyan #6fc2ef before the
  2026-05-08 swap") so the audit trail stays in the spec.
- card_face_svg_pin.rs: rebaselined. Exactly one hash drift
  (back_0 — the canonical Terminal back's badge changed
  colour). Other 56 hashes identical (face SVGs don't
  reference the accent; back_1..4 use unchanged accents). The
  one-hash-drift signal confirms the change scope was
  surgical.

Workspace clippy + cargo test --workspace clean, 1184 passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/splash_plugin.rs

@@ -1,7 +1,7 @@
 //! Launch splash overlay.
 //!
 //! On app start the engine spawns a fullscreen, high-Z overlay that
-//! reads the Terminal-style "boot screen" — a cyan cursor block, the
+//! reads the Terminal-style "boot screen" — an accent-coloured cursor block, the
 //! "Solitaire Quest" wordmark, a short fixture boot log, a progress
 //! bar, and a footer with the design-system palette swatches and the
 //! build version. The overlay fades in over 300 ms, holds for ~1 s,
@@ -470,8 +470,8 @@ fn spawn_check_row(parent: &mut ChildSpawnerCommands, line_font: &TextFont, labe
 
 /// "▌ ready_" line — visual signature of "boot complete, awaiting
 /// input". The leading `▌` glyph picks up `TEXT_PRIMARY` rather than
-/// `ACCENT_PRIMARY` so it doesn't compete with the big cyan cursor in
-/// the header; the *trailing* 6×12 px cyan pulse Node ([`SplashCursorPulse`])
+/// `ACCENT_PRIMARY` so it doesn't compete with the big accent cursor in
+/// the header; the *trailing* 6×12 px accent pulse Node ([`SplashCursorPulse`])
 /// is what carries the "alive, blinking" signal called for by the
 /// mockup. The pulse's alpha is multiplied with the global fade
 /// timeline by [`pulse_splash_cursor`] so it never fights the
@@ -492,7 +492,7 @@ fn spawn_ready_row(parent: &mut ChildSpawnerCommands, line_font: &TextFont) {
                 line_font.clone(),
                 TextColor(transparent(TEXT_PRIMARY)),
             ));
-            // Trailing 6×12 cyan pulse cursor. Node-with-explicit-
+            // Trailing 6×12 accent pulse cursor. Node-with-explicit-
             // dimensions rather than a `█` text glyph so the size
             // doesn't drift with the line font; matches the mockup's
             // 6×12 px spec literally. Pulse animation lives in
@@ -511,7 +511,7 @@ fn spawn_ready_row(parent: &mut ChildSpawnerCommands, line_font: &TextFont) {
 }
 
 /// Progress bar — a 1 px tall track in `BORDER_SUBTLE` with a 100 %-
-/// width cyan fill, plus a `DONE · 247 ASSETS` caption right-aligned
+/// width accent fill, plus a `DONE · 247 ASSETS` caption right-aligned
 /// below. The "247" is fixture text; the bar is decorative, not a
 /// real progress signal. Capped at 720 px width on desktop.
 fn spawn_progress_bar(parent: &mut ChildSpawnerCommands, line_font: &TextFont) {